The present invention relates in general terms to the production of metal or light alloy castings and more specifically applies to the production of members of this type constituted by a metal matrix reinforced by fibres and which have excrescences or outgrowths, such as fixing tabs, lobes, etc.
In the production of light alloy castings, important advances have made it possible to obtain distinctly improved mechanical characteristics. One of the solutions used consists of reinforcing the metal matrixes by incorporating fibres with strong mechanical characteristics. The most widely used fibres are those of alumina, boron, carbon, silicon carbide, silicon, ceramic, etc. This leads to a significant improvement in the mechanical characteristics at ambient temperature, but with a limited elongation at rupture, whilst there is also a definite improvement to the thermal behaviour. Thus, in the case of alloy 2024, the introduction of 30% of short silicon carbide fibres leads to a tensile strength at 350.degree. C. identical to that obtained at 250.degree. C. with the same metal, but not reinforced by fibres. The latter can be in the form of long, continuous fibres, which can be woven or wound, in the form of short fibres, whose length can vary from a few micrometers to a few centimeters, or in the form of particles, such as ground, equiaxial polycrystals, whose diameter is approximately 2 to 3 micrometers, or crystal whiskers in the form of monocrystals of diameter 0.1 to 2 micrometers and a length generally below 100 micrometers.
The characteristics obtained and the technologies used differ according to the type of fibre used as the reinforcement. Thus, in the case of reinforcements constituted by uniformly distributed short fibres average non-oriented characteristics are obtained, but it is possible to obtain small spheres which could then be transformed by deformation (forging, forming, drawing, etc.). However, in the case of reinforcements constituted by continuous fibres, they do not permit any subsequent transformation which would lead to fibre break and instead lead to high characteristics which can also be oriented in accordance with privileged directions.
There are at present several methods for obtaining long fibres. For example, it is possible to metallize a layer of fibres, then heat it and compress it, or extrude a profile or section into which is introduced, during drawing, continuous fibres. French specification No. 2 363 636 describes a higher performance and more easily industrializable method, which consists of casting or pouring the matrix into a previously made fibrous preform which is placed in the mould before casting. Different methods can be used for producing such preforms. For example, it is possible to superimpose fabrics, layers or gauzes inclined in a different manner, or wind a certain number of filaments onto a core, if the part is of revolution. It is also possible to use a multidimensional woven preform.
The structures obtained with such methods generally have remarkable properties both at ambient temperature and at temperatures of approximately 300.degree. to 350.degree. C. These mechanical properties are superior to those of the metal matrix used. These methods lead to simple parts of constant thickness, for which the production of fibrous preforms is easy and the corresponding technology well adapted. However, parts having a simple shape only have limited uses. The sphere of use of the light alloy castings is increased by the crude casting of parts which, apart from a simple general surface, have zones with specific functions, i.e. various bosses or lobes, fixing tabs, connecting zones, etc., i.e. important local outgrowths.
With the methods defined hereinbefore, the continuous fibre preform constituting the general reinforcement of the casting cannot easily be adapted to the strengthening of the local outgrowths. Thus, in the final part, the zones not reinforced by fibres only have the same performance characteristics as the basic matrix. Thus, they constitute the same number of weak points and this is particularly prejudicial, because such outgrowths often serve as connecting or fixing means.